Molecular and Empirical Formulas

Atoms are the basic building blocks of matter. However, a lone atom (an individual unbonded atom) is rarely found in nature. Noble gases, such as helium and neon, are found as lone atoms because they are already stable due to their full valence shell. Other elements are found as molecules or ions. An example of a molecular chemical substance is hydrogen gas. Each molecule of hydrogen gas is composed of two hydrogen atoms bonded to each other. The chemical formula for a hydrogen molecule is H₂, with the subscript indicating the two hydrogen atoms. Molecules that consist of atoms of only one element are called diatomic molecules, also known as elemental molecules.

Other examples of diatomic molecules are oxygen (O₂), nitrogen (N₂), chlorine (Cl₂), fluorine (F₂), bromine (Br₂), and iodine (I₂). Molecular compounds consist of atoms of two or more different nonmetal elements. An example of a molecular compound is water (H₂O), composed of two hydrogen atoms and an oxygen atom. Another example is carbon dioxide (CO₂), composed of one carbon atom and two oxygen atoms.

A molecular formula is a formula that indicates the number of atoms of each type of element in a molecule. The molecular formula for water, for example, is H₂O. The number two following the symbol for hydrogen indicates two hydrogen atoms. The symbol for oxygen does not have a number following it. Lack of a subscript number means there is only one of that atom in the molecule. Another example is butane, a colorless gas often used as fuel. A molecule of butane consists of 4 carbon atoms and 10 hydrogen atoms, so the molecular formula of butane is C₄H₁₀.

An empirical formula is a formula indicating the ratios of elements found in a compound in the lowest whole number possible. For example, the molecular formula of butane is C₄H₁₀, but its empirical formula is C₂H₅ because the ratio of carbon to hydrogen atoms is 2 to 5. Hydrogen peroxide, which has the molecular formula H₂O₂, has the empirical formula HO. The molecular formula and empirical formula of water are both H₂O because the molecular formula of water already shows the elements in the lowest whole-number ratio. Empirical formulas are less informative than molecular formulas because they do not express the exact number of atoms for a molecule.

Molecular mass is the sum of the atomic weights of all atoms in a molecule. When calculating molecular mass, it is important to use the molecular formula of a substance, not its empirical formula. The molecular mass of ethane (C₂H₆), for example, is two times the atomic mass of carbon plus six times the atomic mass of hydrogen: ${2(12.01\;\rm{amu})\;+\;6(1.01\;\rm{amu})\;=\;30.08\;\rm{amu}}$